Method for making titanium dioxide pigment grind dispersion and paint

ABSTRACT

Titanium dioxide may be more efficiently used in a coating composition containing a composite pigment particle-forming polymer if during composite formation, the polymer is initially present in a low concentration and the titanium dioxide particles and newly-formed composite particles are dispersed in a waterborne emulsion polymer that does not form such a composite. A pigment grind dispersion of titanium dioxide pigment-polymer composite particles may be made by mixing titanium dioxide particles with the waterborne emulsion polymer. The composite-forming polymer is then added to the waterborne emulsion polymer, or is added independently and concurrently with the titanium dioxide particles. The resulting titanium dioxide pigment-polymer composite particles provide improved coating composition opacity compared to a coating composition that does not contain such composite particles. The coating compositions can contain reduced titanium dioxide amounts and less than a saturation level of the composite-forming polymer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national stage filing under 35 U.S.C. §371 of International Application No. PCT/US2013/037608 filed Apr. 22, 2013, which claims priority under 35 U.S.C. §119 to U.S. Provisional Application No. 61/790,103 filed Mar. 15, 2013 and 61/636,571 filed Apr. 20, 2012, the disclosures of all of which are incorporated herein by reference.

FIELD

This invention relates to pigmented paints and other coating compositions.

BACKGROUND

Titanium dioxide is a widely-used but increasingly expensive pigment in paints and other coating compositions. In many paint formulations titanium dioxide represents the single most expensive raw material. A number of techniques and ingredients have been suggested for reducing the amount of titanium dioxide in coating composition formulations while still providing acceptable light scattering or opacity. One such ingredient is EVOQUE™ Pre-Composite Polymer supplied by Dow Chemical Company. EVOQUE products are presently available in four grades (EVOQUE 1140, EVOQUE 1180, EXP-4340 and EXP-4463) that are said to have differing degrees of reactivity towards titanium dioxide. According to its supplier, EVOQUE pre-composite polymer “combines with TiO₂ to form a polymer-pigment composite which improves both the wet and dry hiding efficiency of the pigment.”

Also according to its supplier, “A minimum level of EVOQUE Pre-Composite Polymer is required to fully cover the surface of TiO₂ in order to improve hiding efficiency and allow for developing stable formulations.” The recited minimum level is sometimes called the “saturation level” or “saturation level of adsorption” (defined below). The saturation level will depend on the pigment volume concentration (PVC, also defined below), with lesser polymer addition levels being needed to reach the saturation level of adsorption at low PVC and greater polymer addition levels being needed to reach the saturation level of adsorption at high PVC.

Some patents describing composite pigment particle-forming polymers that are believed to be like the polymer present in EVOQUE Pre-Composite Polymer say that other copolymers may be present during formation of the composite particle, so long as such other copolymers do not inhibit or substantially interfere with adsorption of the polymer particle to the pigment particle.

The currently recommended method for making latex paints using EVOQUE Pre-Composite Polymer is carried out by dispersing titanium dioxide into a mixture of EVOQUE Pre-Composite Polymer, water and defoamer to form a composite, and then combining the composite with the remaining paint ingredients including an acrylic latex paint binder in order to form a finished paint formulation. In this method, the acrylic latex paint binder is not combined with the composite until after the composite has been formed. The resulting paint or other coating composition may attain a given degree of pigment hiding at a somewhat reduced titanium dioxide pigment level, or may attain a somewhat increased degree of pigment hiding at a given titanium dioxide pigment level. However, the potential savings in titanium dioxide raw material costs and the potential benefits in pigment hiding efficiency are offset by the EVOQUE product cost.

From the foregoing, it will be appreciated that what remains needed in the art are improved titanium dioxide-containing coating compositions having even lower raw material cost, even greater pigment hiding efficiency, or both lower cost and greater efficiency. Methods for the manufacture of such compositions are disclosed and claimed herein.

SUMMARY OF THE INVENTION

We have found that paints and other coating compositions having excellent opacity and hiding power at low usage levels of both titanium dioxide and composite pigment particle-forming polymer may be obtained by disregarding the above-mentioned supplier's requirements and instructions. Without intending to be bound by theory, we believe that titanium dioxide may be more efficiently used in a coating composition containing a composite pigment particle-forming polymer if during composite formation, the composite pigment particle-forming polymer is initially present in a low concentration and the titanium dioxide particles and newly-formed composite particles are dispersed in a waterborne emulsion polymer that does not form such a composite. This can if desired also enable use of less than a saturation level of the composite-forming polymer while still providing composite particles that efficiently employ titanium dioxide. By using a latex paint binder or other low-cost waterborne emulsion polymer as the interfering polymer, an overall cost reduction can be achieved together with equivalent or even improved hiding and other paint properties.

The present invention provides, in one aspect, a method for making a pigment grind dispersion of titanium dioxide pigment-polymer composite particles, which method comprises:

-   -   (a1) mixing titanium dioxide particles with an aqueous emulsion         containing water-insoluble film-forming first polymer particles         that do not by themselves form composite particles with the         titanium dioxide particles, and then     -   (b1) adding an aqueous emulsion containing water-insoluble         film-forming second polymer particles that adsorb to the         titanium dioxide particles,         or     -   (a2) circulating an aqueous emulsion containing water-insoluble         film-forming first polymer particles that do not by themselves         form composite particles with titanium dioxide particles, and     -   (b2) independently and concurrently adding to such first polymer         particles (i) a dispersion of titanium dioxide particles         and (ii) an aqueous emulsion containing water-insoluble         film-forming second polymer particles that adsorb to the         titanium dioxide particles,         thereby forming titanium dioxide pigment-polymer composite         particles that provide improved coating composition opacity         compared to a coating composition that does not contain such         composite particles.

The disclosed methods provide dried coatings having excellent opacity and hiding power at reduced titanium dioxide and composite-forming polymer usage levels. The disclosed methods enable use of a second polymer particle amount that is less than the saturation level of adsorption when another polymer is not present. The disclosed methods and pigment grind dispersions appear to have particular value for making semigloss, gloss, high hiding or super hiding paints and other coating compositions. The disclosed method also permits the late-stage corrective addition of a composite-forming polymer to a coating composition which has been found during manufacture to have low tint strength, low hiding power or other deficiencies that might otherwise be addressed by starting a new batch containing a greater level of titanium dioxide. If needed such a corrective addition could even be performed after all other coating composition ingredients had already been dispersed into the coating composition, thereby providing an important corrective step in large-scale mixing of paints and other coating compositions.

DETAILED DESCRIPTION

The recitation of a numerical range using endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).

The terms “a,” “an,” “the,” “at least one,” and “one or more” are used interchangeably. Thus, for example, a coating composition that contains “an” additive means that the coating composition includes “one or more” additives.

The terms “adsorb” and “adsorption” when used with respect to titanium dioxide particles and polymer particles in a fluid medium means that the titanium dioxide and polymer particles when allowed to equilibrate in the fluid medium and observed using electron microscopy appear to have formed stable composite particles in which a sufficient number of titanium dioxide particles contact a sufficient number of polymer particles so as to provide an improvement in titanium dioxide particle spacing and contrast ratio in a dried coating composition containing such composite particles. If observed using scanning electron microscopy, the fluid medium may be evaporated away and the sample subjected to conventional plating and polishing steps prior to observation. If observed using transmission electron microscopy, the fluid medium may be drawn down to a thin film and frozen prior to observation. The observed adsorption may arise due to physisorption, chemisorption, electrostatic attraction or by other means.

The term “architectural paint” means a coating composition for use on interior or exterior building components, and includes both paints and stains.

The term “binder” means a film-forming natural or synthetic polymer suitable for use in paints and other coating composition.

The term “circulating” when used with respect to a composition means that the composition is a fluid that may or may not contain dispersed solid particles and which is undergoing stirring, agitation or other shear forces sufficient to mix components in the composition.

The term “contrast ratio” means a value determined by casting a 51 μm (2 mil) dry thickness coating film over the white and black portions of a Leneta Co. Form 3B opacity drawdown chart (from BYK-Gardner USA), measuring L* as defined in the ASTM International Standards on Color and Appearance Measurement: 8th Edition, and dividing the L* value measured over the black portion by the L* value measured over the white portion.

The terms “titania composite ratio” or “TCR” when used with respect to a pigment grind dispersion, paint or other coating composition mean the total percentage of the dried solids volume, excluding extender pigments and opaque polymers, occupied by titanium dioxide particles in such dried solids.

The term “film-forming” when used in reference to a water-insoluble particulate polymer means that an aqueous dispersion of the polymer particles can be formed, coated in a thin wet layer (e.g., of about 25 μm thickness) on a suitable support, and dried or otherwise hardened (optionally with the aid of a suitable coalescent) to form a substantially continuous coating over the support.

The term “fluid” when used in reference to a substance means that the substance is a liquid that may or may not contain dispersed solid particles and which has a loss modulus (G″) greater than its storage modulus (G′) and a loss tangent (tan δ) greater than 1.

The term “low VOC” when used with respect to paints and other coating composition means that the coating composition contains less than about 5 wt. percent volatile organic compounds, preferably less than about 3 wt. % volatile organic compounds, more preferably less than about 1 wt. % volatile organic compounds and most preferably less than about 0.5 wt. % volatile organic compounds based upon the total coating composition weight.

The term “opaque” when used with respect to paints and other coating composition means that a dry film of the coating composition has a contrast ratio greater than 95% at a 51 μm (2 mil) dry film thickness.

The term “paint” means a coating composition including pigment and a film-forming binder which when applied to form a thin (e.g., 100 μm) wet thickness coating film on a freshly-sanded smooth wood surface, will dry to form a continuous film over the surface, and includes non-penetrating or other stains that will dry to form such a continuous film.

The term “pigment” means an inorganic particulate material having light-reflective characteristics and a surface energy and particle size suitable for use in paints and other coating compositions.

The terms “pigment volume concentration” or “PVC” when used with respect to a pigment grind dispersion, paint or other coating composition mean the total percentage of dried solids volume, including extender pigments and opaque polymers, occupied by a recited pigment species (or if no pigment species is recited, then by all pigment species) in such dried solids.

The terms “polymer” and “polymeric” include polymers as well as copolymers of two or more monomers.

The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.

The term “primer” refers to a coating composition that is applied in one or more layers to a bare substrate and which if left uncoated without a topcoat would not be capable of withstanding extended outdoor exposure (e.g., exposure equivalent to one year of vertical south-facing Florida sunlight) without visually objectionable deterioration.

The terms “saturation level” and “saturation level of adsorption” when used with respect to titanium dioxide particles and polymer particles that adsorb to the titanium dioxide particles in a fluid medium mean a polymer addition level at which the amount of polymer adsorbed per unit of titanium dioxide particles plateaus.

The term “solvent-borne” when used with respect to paints or other coating compositions means that the major liquid vehicle or carrier for such coating composition is a nonaqueous solvent or mixture of nonaqueous solvents.

When used with respect to a component which may be found in a paint or other coating composition, the term “substantially free of” means containing less than about 1 weight percent of the component based on the coating composition weight.

The term “topcoat” refers to a coating composition which when dried or otherwise hardened provides a decorative or protective outermost finish layer on a coated substrate. By way of further explanation, such topcoats may be applied in one or more layers and may be applied to bare or primer-coated substrates.

The term “water-borne” when used with respect to paints and other coating compositions means that the major liquid vehicle or carrier for such coating composition is water.

The term “water-insoluble” when used with respect to a polymer means that particles of the polymer may be mixed (optionally with the aid of a suitable dispersant) into water to form a particulate polymer dispersion.

The disclosed titanium dioxide particles provide light scattering sites within a dried film of the disclosed coating compositions, and impart hiding or opacity to the dried film. Representative titanium dioxide particles can have a variety of forms, including rutile, anatase and Brookite titanium dioxide. Blends, for example of rutile and anatase titanium dioxide, may also be employed, as well as blends of titanium dioxide together with pigments other than titanium dioxide. Representative lists of such other pigments may be found in U.S. Pat. No. 5,509,960 (Simpson et al.) and U.S. Pat. No. 7,179,531 B2 (Brown et al.), and in Published U.S. Patent Application No. US 2011/0290152 A1 (Gebhard et al. '152). The titanium dioxide particles may have a variety of shapes and sizes but desirably will scatter photons having wavelengths in the spectral region from 300 nm to 750 nm and especially in the visible spectral region from 380 nm to 700 nm. Suitable shapes for the titanium dioxide particles include spherical shapes, such as a regular sphere, an oblate sphere, a prolate sphere, and an irregular sphere; cubic shapes such as a regular cube and a rhombus; plate-like shapes including a flat plate, a concave plate, and a convex plate; and irregular shapes. Particles having spherical shapes desirably have average diameters of about 10 nm to about 1,000 nm, e.g., about 100 nm to about 500 nm, about 200 nm to about 300 nm, or about 230 nm. Particles having non-spherical shapes desirably have a maximum diameter of up to about 1 micrometer, e.g., up to about 500 nm or up to about 300 nm. The titanium dioxide particles may include one or more coatings, e.g., of silica, alumina, zirconia or combinations thereof such as a silica coating and an alumina coating. Exemplary commercially available titanium dioxide particles include those made by the chloride process and those made by the sulfide process, and those made in slurry or dry forms, e.g., KRONOS™ 1071, 2020, 2044, 2090, 2101, 2102, 2131, 2160, 2210, 2310, 4102, 4310 and 4311 from Kronos, Inc., TIONA™ 595 and 596i from Millennium Specialty Chemicals Inc. TIPURE™ TS-6200, R-706, R-741, R-746, R-900, R-902+, R-931 and R-960 from E. I. duPont de Nemours and Company, TRONOX™ CR-813, CR-813S, CR-826, CR-826S, CR-828, CR-834 and CR-880 from Tronox Corporation, and products from other suppliers including Bluestar New Chemical Materials Co., Ltd., Hebei Chuanghui Chemicals Co., Ltd., Henan Billions Chemicals Co., Ltd., Ishihara Sangyo Kaisha, Ltd., Nanjing Hengsiman Chemical Co., Ltd., Pangang Titanium Industry Co., Ltd., Qingdao Gracecorp Co., Ltd., Sakai Chemical Industry Co., Ltd., Shanghai Yuejiang Titanium Chemical Manufacturer Co., Ltd., Shijiazhuang Kelichuangxin Chemicals Co., Ltd. and Xuzhou. Zhonglian Chemical Technology Co., Ltd. and mixtures thereof.

The disclosed methods may employ a variety of water-insoluble insoluble film-forming first polymer particles (the “first polymer”) that do not by themselves form composite particles with (for example, do not adsorb to) the titanium dioxide particles. Exemplary first polymers typically are waterborne addition polymers formed by the aqueous emulsion polymerization of one or more ethylenically unsaturated monomers that do not contain pendant groups sufficient to cause interfering adsorption of the first polymer particles to titanium dioxide particles. Exemplary first polymers include acrylic emulsions, ethylene vinyl acetate emulsions, polybutadiene emulsions, polyvinylidene emulsions, styrene acrylic emulsions, vinyl acrylic emulsions and vinyl acetate-vinyl versatate emulsions and mixtures thereof, with latex emulsions (e.g., acrylic latex emulsions) being preferred. The first polymer emulsions normally contain at least the first polymeric particles, water, and one or more emulsifiers. The first polymer particles may include one or more functional groups capable of reacting with an external crosslinker, and such external crosslinker may also be a part of the disclosed coating compositions. For example, the first polymer particles may include hydroxyl-functional groups capable of reacting with an amino resin or polyisocyanate crosslinker. Exemplary such amino resins include waterborne coating-compatible melamine, urea and glycoluril crosslinkers available from suppliers such as Cytec Industries Inc., including CYMEL™ 328 and CYMEL 383 aminoplast resins. Exemplary such polyisocyanate crosslinkers include waterborne coating-compatible polyisocyanate crosslinkers available from suppliers such as Bayer MaterialScience, including BAYHYDUR™ 304 and BAYHYDUR 3100 polyisocyanates. The first polymer particles may also include functional groups capable of reacting with a waterborne coating-compatible polyepoxide crosslinker, or epoxy-functional groups capable of reacting with a waterborne coating-compatible epoxy curative. Exemplary such epoxy curatives include ANQUAMINE™ 721 water-reducible epoxy curative from Air Products and Chemicals, Inc. and BECKOPOX™ EH 2179W/65WA water-reducible epoxy curative from Cytec Industries, Inc. The first polymer particles may also or instead include one or more functional groups capable of undergoing an internal crosslinking reaction (viz., a so-called self-crosslinking polymer), or may harden without requiring a crosslinking or other curing reaction for dry film formation to take place shortly after application. In some embodiments it will be desirable to employ a softer first polymer that does not require a volatile coalescent for film formation, and to employ an internal or external crosslinking agent to promote film formation. Exemplary first polymers may be prepared as described for example in U.S. Pat. No. 8,293,361 B2 (Killilea et al.) or obtained from a variety of commercial sources. Exemplary commercially available first polymers include ALBERDINGK AC 2514, ALBERDINGK AC 25142, ALBERDINGK AC 2518, ALBERDINGK AC 2523, ALBERDINGK AC 2524, ALBERDINGK AC 2537, ALBERDINGK AC 25381, ALBERDINGK AC 2544, ALBERDINGK AC 2546, ALBERDINGK MAC 24, and ALBERDINGK MAC 34 emulsions from Alberdingk Boley, Inc.; AQUAMAC 720 emulsion from Hexion Specialty Chemicals; EPS 2538 acrylic latex, EPS 2540 styrene acrylic latex, EPS 2771 acrylic emulsion and EPS 2725 acrylic latex emulsions from EPS Corp.; RESYN™ 7305 vinyl acrylic emulsion from Celanese Emulsion Polymers; RHOPLEX™ 3131-LO, RHOPLEX™ AC-1020, RHOPLEX E-693, RHOPLEX E-940, RHOPLEX E-1011, RHOPLEX E-2780, RHOPLEX HG-95P, RHOPLEX HG-700, RHOPLEX HG-706, RHOPLEX PR-33, RHOPLEX TR-934HS, RHOPLEX TR-3349, RHOPLEX VSR-1050 and RHOPLEX VSR-2015 acrylic emulsions from Rohm and Haas Co.; RHOSHIELD™ 636, RHOSHIELD 3188 and RHOSHIELD 3275 emulsions from Rohm and Haas Co.; JONCRYL™ 538, JONCRYL 1552, JONCRYL 1972, JONCRYL 1980, JONCRYL 1982, JONCRYL 1984, JONCRYL 1987 and JONCRYL 8383 acrylic emulsions from BASF Resins; NEOCRYL™ A-1127, NEOCRYL A-6115, NEOCRYL XK-12, NEOCRYL XK-90, NEOCRYL XK-98 and NEOCRYL XK-220 acrylic latex polymers from DSM NeoResins, Inc., and mixtures thereof.

A variety of water-insoluble film-forming second polymer particles (the “second polymer”) that adsorb to titanium dioxide particles may be employed in the disclosed pigment grind dispersions and the disclosed coating compositions. Exemplary second polymers are described in the above-mentioned Simpson et al. and Brown et al. patents as well as in U.S. Pat. No. 5,385,960 (Emmons et al. '960), U.S. Pat. No. 6,080,802 (Emmons et al. '802), U.S. Pat. No. 6,576,051 B2 (Bardman et al. '051), U.S. Pat. No. 6,881,782 B2 (Crater et al.), U.S. Pat. No. 7,081,488 B2 (Bardman et al. '488), U.S. Pat. No. 7,265,166 B2 (Gebhard et al. '166) and U.S. Pat. No. 8,318,848 B2 (Finegan et al.), in European Patent Application No. EP 0 700 976 A2 (Tioxide Group Services Limited) and in International Application No. WO 93/11181 (Emmons et al. '181). The second polymers typically are waterborne addition polymers formed by the aqueous emulsion polymerization of at least one ethylenically unsaturated monomer containing a pendant group capable of facilitating adsorption of the second polymer particles to titanium dioxide particles, and optionally one or more other ethylenically unsaturated monomers. Exemplary such pendant groups include phosphorus groups (e.g., dihydrogen phosphate groups), acetoacetoxy groups, 1,3-dicarbonyl groups, aldehyde groups, acid groups, amine groups, epoxy groups, isocyanate groups, thiorane groups, isothiocyanate groups, alcohol groups, carbodiimides groups, aziridine groups, haloalkane groups, and halophenyl groups. In place of or in addition to the at least one ethylenically unsaturated monomer containing such a pendant group, the second polymer may be formed from a reaction mixture containing one or more ethylenically unsaturated monomers containing a select pendant group capable of reacting with a coupling agent that will provide a group capable of adsorbing the second polymer particle to a titanium dioxide particle, followed by reaction of the resulting second polymer particles with such a coupling agent. Exemplary select pendant groups and coupling agents are described in, e.g., the above-mentioned Bardman et al. '488 and Brown et al. patents. The second polymer particles may be single stage or multistage polymers. Exemplary multistage polymers are described in, e.g., the above-mentioned Emmons et al. '960, Emmons et al. '802, Bardman et al. '051, Bardman et al. '488, Brown et al. and Finnegan et al. patents. The glass transition temperature (Tg) of the second polymer or the Tg of each stage in a multistage second polymer may be adjusted to obtain desired properties. For example, a second polymer or second polymer stage having a higher Tg (for example, a Tg of about 30 to about 230° C.) may be employed to adjust finished film properties such as film hardness or scrub resistance. A second polymer or second polymer stage having a lower Tg (for example, a Tg of about −65 to about 30° C.) may be employed to adjust film formation properties such as the minimum film-forming temperature (MFFT), or to make the second polymer self-coalescing without requiring a coalescent. The second polymer particles may include vesicles or other voids, or may be free of voids. Exemplary commercially available second polymers include EVOQUE 1140, EVOQUE 1180, EXP-4340 and EXP-4463 pre-composite polymer from Dow Chemical Company, FASTRACK HE-2706 high efficiency binder from Dow Chemical Company and mixtures thereof.

The disclosed pigment grind dispersions contain (and the disclosed methods may employ) water, which may be tap, deionized, distilled, reverse osmosis or recycled water. Preferably the pigment grind dispersion contains sufficient water to facilitate efficient pigment grind mixing (e.g., about 20 to about 80 weight percent water based on the weight of ingredients in the pigment grind dispersion), and a final coating composition made from such a dispersion preferably contains sufficient water so that about 20 to about 80 weight percent solids and more preferably about 35 to about 65 weight percent solids are present when the composition is applied to a substrate.

The ingredient amounts in the disclosed pigment grind dispersions will to some extent depend upon the other ingredients present and may be empirically determined. As an example, the titanium dioxide PVC level and desired film former binder amounts in the final coating composition may be used to estimate initial ingredient amounts in the pigment grind dispersion. In general, higher PVC levels will require larger first and second polymer amounts in the pigment grind dispersion. When the first and second polymers are the only film formers in the final coating composition, then increases in the first polymer amount may be offset by decreases in the second polymer amount, and vice versa. The second polymer particle amount may if desired be less than the saturation level of adsorption when another polymer is not present. The second polymer saturation level of adsorption may be determined using techniques such as microscopy or viscosity studies on the pigment grind dispersion to establish the level at which the amount of second polymer adsorbed per unit of titanium dioxide particles plateaus in the absence of the first polymer.

As a further guide, a formulator may determine the amounts of titanium dioxide particles and first polymer needed to achieve a finished paint or other coating composition with desired PVC and hiding power characteristics without using the second polymer. A second polymer amount is then chosen to provide efficient use of titanium dioxide particles and the desired hiding power at less than or up to the saturation level. As a general guide for use with an aqueous second polymer emulsion containing about 45 wt. % solids and a titanium dioxide shiny containing about 75 wt. % solids, the second polymer may be employed at a wet weight (viz., the weight of a dispersion including its solids and vehicle) corresponding to about 50 to about 200 percent of the titanium dioxide particle wet weight, with amounts around 50 to 150 percent, 50 to 100 percent or 60 to 80 percent being preferred. These amounts may be adjusted as needed for use with second polymer emulsions containing other solids levels (for example, solids levels of about 30 to 60 or at about 40 to 50 wt. %) and titanium dioxide slurries containing other solids levels (for example, solids levels of about 40 to 85 or about 50 to about 75 wt. %). The first polymer amount may be reduced by an amount corresponding to the second polymer volume solids, so as to maintain a constant PVC level in the starting and adjusted coating compositions. The titanium dioxide amount may for example be sufficient to provide a pigment dispersion having a titanium dioxide PVC of at least about 20, at least about 21, at least about 22, at least about 23, at least about 24, at least about 25, at least about 26, at least about 27 or at least about 30, and up to about 50, up to about 45 or up to about 40. The selected titanium dioxide level may for example also be sufficient to provide a pigment grind dispersion having a TCR of at least about 30, at least about 31, at least about 32, at least about 33, at least about 35, at least about 37, or at least about 40, and up to about 60, up to about 50 or up to about 45. Somewhat higher PVC levels may be desirable at lower TCR levels, and somewhat lower PVC levels may be desirable at higher TCR levels. The disclosed method and pigment grind dispersion can provide improved storage stability (for example, avoidance of undesirable changes in viscosity, such as rapid viscosity increases), reduced grit (viz., solid particle) formation, or both improved storage stability and reduced grit formation compared to pigment grind dispersions prepared using previously-recommended methods. Improved storage stability, reduced grit formation or both improved storage stability and reduced grit formation are useful for a variety of reasons including facilitating inventory management, process control and process improvement in large-scale mixing of paints and other coating compositions. The pigment grind dispersion in the finished coating composition formulation preferably will contain about 27 to about 37 weight percent titanium dioxide particles, about 55 to about 14 weight percent first polymer particles, and about 18 to about 49 weight percent second polymer particles, all based on the total weight of ingredients in the pigment grind dispersion.

The disclosed pigment grind dispersions may include optional ingredients that assist with or do not unduly interfere with pigment dispersion, such as extender particles, dispersing agents, defoamers, wetting agents and mixtures thereof. Exemplary extender particles may for example have an index of refraction similar to that of a dried film of the second polymer, and may for example not significantly scatter visible light or may flatten the dried coating appearance. The extender particle index of refraction may for example be less than 1.8 and greater than or equal to 1.3. The extender particles may for example be small particles having an average particle diameter less than or equal to twice the average particle diameter of the titanium dioxide pigment-polymer composite particles, or large particles having an average particle diameter greater than twice the average particle diameter of the titanium dioxide pigment-polymer composite pigment particles. Exemplary extender particles include calcium carbonate, calcium sulfate, barium sulfate, mica, clay, calcined clay, feldspar, nepheline, syenite, wollastonite, diatomaceous earth, alumina silicates, non-film forming polymer particles, aluminum oxide, silica, talc, mixtures thereof and other materials that will be familiar to persons having ordinary skill in the art.

Exemplary dispersing agents include anionic polyelectrolyte dispersants such as maleic acid copolymers, acrylic acid copolymers including methacrylic acid copolymers, and carboxylic acids such as tartaric acid, succinic acid, citric acid, itaconic acid, mixtures thereof and other materials that will be familiar to persons having ordinary skill in the art.

Exemplary defoamers include silicones, ethylene oxide propylene oxide copolymers, oils and waxes such as FOAMSTAR™ A-32, FOAMSTAR A-34, FOAMSTAR A-36, FOAMSTAR A-38, FOAMSTAR A-39, FOAMASTER™ 111, FOAMASTER 333 and FOAMASTER SA-3 from Cognis, TEGO™ FOAMEX™ 810 from Evonik, mixtures thereof and other materials that will be familiar to persons having ordinary skill in the art, including products from other suppliers such as Air Products and Chemicals, Ashland, BASF, BYK-Gardner USA, Cytec, Rhone Poulenc and Troy Corporation.

The amounts of such optional pigment grind dispersion ingredients will normally be empirically determined. For example, the pigment grind dispersion may contain from zero to more than 100 weight percent extender pigment based on the weight of titanium dioxide pigment. Depending on the type of dispersing agent, the dispersing agent amount may for example be about 0.3 to about 5 weight percent based on the total pigment weight in the pigment grind dispersion.

Other ingredients that may be included in the pigment grind dispersion include cosolvents, wetting agents, surfactants, biocides, and pigments other than titanium dioxide. Normally however such other ingredients will be added during a subsequent stage rather than being added to the pigment grind dispersion.

The disclosed composite particles may be prepared by circulating the disclosed first polymer emulsion and if need be additional water in a suitable mixer. Next, titanium dioxide particles are added to the mixer in a dry form or preferably in a suitable dispersion (e.g., a slurry), followed (after the titanium dioxide particles have become well dispersed in the first polymer emulsion) by addition of the second polymer particles. Alternatively, a dispersion (e.g., a slurry) of the titanium dioxide particles and an aqueous emulsion of the second polymer particles are independently and concurrently added to the mixer. The additions desirably are gradual and controlled to minimize grit formation. The resulting mixture of titanium dioxide particles, first polymer particles and second polymer particles is in any event desirably subjected to further mixing or otherwise circulated to allow sufficient time for the second polymer particles and titanium dioxide particles to form the disclosed composite particles. The required time will depend upon a variety of factors including the chosen titanium dioxide particle type, amount and surface treatment or coating if any; the respective types and amounts of first and second polymer particles; the optional dispersant type and amount; the circulation method and shear rate; and temperature. The time required for attainment of a suitable equilibrium level of composite particle formation may for example be from about 5 minutes up to about 2 hours, with shorter or longer times being necessary or desirable in some instances. The final coating composition is completed by combining the dispersion with the remaining coating composition ingredients. The final coating composition thus includes the above-described pigment grind dispersion together with one or more additional optional ingredients such as film-forming polymers; opacifying non-film-forming copolymers; coalescents; cosolvents or plasticizers; thickeners and other rheology modifiers; pigments other than titanium dioxide; surfactants or dispersants (aside from those which already may be present in the first or second polymers), additional water, and other adjuvants. The additional film-forming polymer desirably is the above-mentioned first polymer but may be other film-forming polymers that will be familiar to persons having ordinary skill in the art. The amounts and types of such additional film-forming polymers may vary widely and typically will be empirically selected.

Exemplary opacifying non-film-forming copolymers polymers include those described in, e.g., U.S. Pat. No. 4,885,320 (Biale), U.S. Pat. No. 5,663,213 (Jones et al.) and U.S. Pat. No. 6,646,058 B1 (Koger), and in Published U.S. Patent Application Nos. US 2009/0162558 A1 (Bardman et al. '558) and US 2010/0166967 A1 (Fasano), as well as commercially available opacifying non-film-forming copolymers such as OPAQUE™ OP-62, OPAQUE OP-96 and OPAQUE™ ULTRA opaque polymers from Dow Chemical Company. When an opacifying non-film-forming copolymer is present, the coating compositions preferably contain about 2 to about 9 weight percent opacifying non-film-forming copolymer based on the final coating composition weight.

Coalescents may assist in coalescing a film-forming emulsion polymer into a continuous film. Exemplary coalescents include benzoates such as alkyl benzoates, monobenzoates and dibenzoates; hexanoates such as OPTIFILM™ 400 tri(ethylene glycol) bis(2-ethylhexanoate) from Eastman Chemical Co.; dioctyl maleate; oleic acid propylene glycol esters such as EDENOL™ EFC-100 from Cognis having the formula HOCH(CH₃)CH₂OC(O)(CH₂)₇CH═CH(CH₂)₇CH₃); UCAR Filmer IBT, UCAR n-propyl propionate, UCAR n-butyl propionate and UCAR n-pentyl propionate from DOW Chemical Co.; and TEXANOL™ ester alcohol from Eastman Chemical Co.; materials discussed in U.S. Pat. No. 6,762,230 B2, U.S. Pat. No. 7,812,079 B2 and U.S. Pat. No. 8,110,624 B2 (collectively, Brandenburger et al.), in U.S. Pat. No. 8,106,239 B2 (Zhou et al.) and U.S. Pat. No. 8,394,496 B2 (Foster et al.) and in U.S. Published Patent Application No. US 2009/0149591 A1 (Yang et al.); mixtures thereof and the like. When a coalescent is present, the coating compositions preferably contain about 0.05 to about 10 or about 0.05 to about 5 weight percent coalescent based on the final coating composition weight.

Cosolvents may assist in mixing or coating the composition; may speed up, retard or otherwise change the time or emissions associated with drying; may improve wet edge properties or overlap characteristics; may improve freeze-thaw protection, or may provide or improve other features, and generally will not be retained in a film of the dried, cured or otherwise hardened coating composition. A chosen cosolvent may be hazardous air pollutant solvent (HAPS material) but preferably is a non-HAPS material or is substantially free of HAPS materials. Exemplary cosolvents may for example include glycols (e.g., ethylene glycol and diethylene glycol), glycol ethers (e.g., DOWANOL™ DPM and Butyl CELLOSOLVE™ from Dow Chemical Co.), alcohols (e.g., n-propanol, isopropanol, n-butanol, isobutanol, 2-methyl butanol, isoamyl alcohol and other primary amyl alcohol isomers, n-pentanol, 2-ethylhexanol, 4-hydroxy-2,6,8-trimethylnonane and diisobutyl carbinol), esters and ester alcohols (e.g., isopropyl acetate; n-butyl acetate; isobutyl acetate; n-propyl acetate; primary amyl acetate mixed isomers, and UCAR™ Ester EEP from Dow Chemical Co.), ketones (e.g., diisobutyl ketone and ECOSOFT™ Solvent IK from Dow Chemical Co.), CARBOWAX™ 300 and CARBOWAX 600 polyethylene from Dow Chemical Co., mixtures thereof and the like. When a cosolvent is present, the coating compositions preferably contain about 0.1 to about 10 or about 2 to about 5 weight percent cosolvent based on the final coating composition weight.

Plasticizers may improve cured film flexibility, or may provide or improve other features, and generally will be retained in a film of the dried, cured or otherwise hardened coating composition. In some formulations a plasticizer may perform functions associated with or may be identified as a coalescent. Exemplary plasticizers include CITROFLEX™ 4 citric acid ester from Vertellus Specialties Inc. and PARAPLEX™ G-30, PARAPLEX G-41, PARAPLEX G-60, PARAPLEX RGA-2 and PARAPLEX WP-1 plasticizers from Dow Chemical Co. When a plasticizer is present, the coating compositions preferably contain about 0.1 to about 10 or about 2 to about 5 weight percent plasticizer based on the final coating composition weight.

Exemplary thickeners and other rheology modifiers include sedimentation inhibitors, hydrophobic ethoxylated urethane resin (HEUR) thickeners, hydrophobically-modified, alkali-soluble or alkali-swellable emulsion (HASE) thickeners), cellulosic thickeners, polysaccharide thickeners and mixtures thereof. Exemplary commercially-available rheology modifiers include NATROSOL™ 250 and the AQUAFLOW™ series from Ashland, ATTAGEL™ 50 from BASF Corp., the CELLOSIZE™ series and UCAR POLYPHOBE™ T-900 and T-901 from Dow Chemical Co., BENTONE™ AD and BENTONE EW from Elementis Specialties, LATTICE™ NTC-61 from FMC Biopolymer and ACRYSOL™ RM-6, ACRYSOL RM-8, ACRYSOL RM-12W and ACRYSOL RM-2020NPR all from Rohm & Haas. When a rheology modifier is present, the coating compositions preferably contain about 0.1 to about 3 or about 0.5 to about 3 weight percent rheology modifier based on the final coating composition weight.

Exemplary other pigments include those described in U.S. Pat. No. 8,141,599 B2 (Korenkiewicz et al.) and U.S. Patent Application Publication No. US 2012/0004359 A1 (Cavallin et al.). The amounts and types of such other pigments may vary widely and typically will be empirically selected to provide a desired coating composition tint.

Exemplary surfactants or dispersants include anionic, amphoteric and nonionic materials. Commercially-available surfactants or dispersants include the TAMOL™ series from Dow Chemical Co., nonyl and octyl phenol ethoxylates from Dow Chemical Co. (e.g., TRITON™ X-45, TRITON X-100, TRITON X-114, TRITON X-165, TRITON X-305 and TRITON X-405) and other suppliers (e.g., the T-DET N series from Harcros Chemicals), alkyl phenol ethoxylate (APE) replacements from Dow Chemical Co., Elementis Specialties, Inc. and others, various members of the SURFYNOL™ series from Air Products and Chemicals, Inc. (e.g., SURFYNOL 104, SURFYNOL 104A, SURFYNOL 104BC, SURFYNOL 104DPM, SURFYNOL 104E, SURFYNOL 104H, SURFYNOL 104PA, SURFYNOL 104PG50, SURFYNOL 104S, SURFYNOL 2502, SURFYNOL 420, SURFYNOL 440, SURFYNOL 465, SURFYNOL 485. SURFYNOL 485W, SURFYNOL 82, SURFYNOL CT-211, SURFYNOL CT-221, SURFYNOL OP-340, SURFYNOL PSA204, SURFYNOL PSA216, SURFYNOL PSA336, SURFYNOL SE and SURFYNOL SE-F), various fluorocarbon surfactants from 3M, E. I. DuPont de Nemours and Co. and other suppliers, and phosphate esters from Ashland, Rhodia and other suppliers. When a surfactant or dispersant is present, the coating compositions preferably contain about 0.1 to about 10 weight percent and more preferably about 1 to about 3 weight percent surfactant or dispersant based on the total composition weight.

The disclosed coating compositions may contain a variety of other adjuvants that will be familiar to persons having ordinary skill in the art. Representative adjuvants are described in Koleske et al., Paint and Coatings Industry, April, 2003, pages 12-86. Exemplary adjuvants and commercial examples of the same include anti-cratering agents; biocides, fungicides, mildewcides and preservatives (e.g., BUSAN™ 1292 from Buckman Laboratories, Inc., NOPCOCIDE™ N-40D from Cognis, KATHON™ LX from Rohm & Haas, and POLYPHASE™ 663, POLYPHASE 678 and POLYPHASE PW-40 from Troy Corporation); curing indicators; heat stabilizers; leveling agents; light stabilizers (e.g., hindered amine light stabilizers such as TINUVIN™ 123-DW and TINUVIN 292 HP from Ciba Specialty Chemicals); optical brighteners; ultraviolet light absorbers (e.g., TINUVIN 234 and TINUVIN 1130 from Ciba Specialty Chemicals); wetting agents (e.g., BYK™ 346 and BYK 348 from Altana, PENTEX™ 99 from Rhodia and TROYSOL LAC™ from Troy corporation); waxes (e.g., AQUACER™ 593 from Altana, HYDROCER™ 303 from Shamrock Technologies, Inc. and MICHEM™ Emulsion 32535 from Michelman, Inc.); and the like. The types and amounts of these and other adjuvants typically will be empirically selected.

The disclosed coating compositions may be packaged in any convenient packaging suitable for storing a desired quantity of the coating composition without premature gelation, undue separation or other undesirable degradation during storage. Exemplary packaging containers include cans, pails, bottles, drums, totes and tanks. The disclosed compositions may be factory-applied to substrates such as building components at a manufacturing site, or may be supplied to end users and applied onsite to finished articles, e.g., as paints for use on decks, siding, roofing or other surfaces.

The coating compositions may be applied to a variety of substrates including metals (including aluminum, brass, copper, iron, pot metal, steel, tin and zinc), woods (including engineered woods, impregnated woods and wood-derived materials), plastics (including thermoplastics and thermosets), composites, and other materials that will be familiar to persons having ordinary skill in the art.

The coating compositions may be applied using a variety of methods that will be familiar to persons having ordinary skill in the art, including spraying (e.g., air-assisted, airless or electrostatic spraying), brushing, roller coating, flood coating and dipping. The compositions may be applied at a variety of wet film thicknesses. Preferably the wet film thickness is such as to provide a dry film thickness of about 13 to about 260 μm (about 0.5 to about 10 mil) and more preferably about 25 to about 75 μm (about 1 to about 3 mil) for the dried coating. The applied coating may be cured by allowing it to air dry or by accelerating curing using a variety of drying devices (e.g., ovens) that will be familiar to persons having ordinary skill in the art. Preferred heating temperatures for curing the coating compositions are about 50° to about 65° C., and more preferably about 60° to about 65° C., and preferred heating times are at least three minutes and less than 60 minutes, less than 45 minutes, less than 30 minutes, less than 15 minutes, less than 10 minutes, less than six minutes or less than five minutes. The heating time will tend to decrease with increased temperature, increased airflow or decreased humidity.

The invention is further illustrated in the following non-limiting examples, in which all parts and percentages are by weight unless otherwise indicated. In several of the examples, tint strength was evaluated using both untinted (base paint) and tinted samples. The tinted samples were prepared by adding 11.1 mL (⅜ fluid ounce) thalo green colorant to 0.47 L (1 pint) of the base paint and mixing the resulting tinted paint on a paint shaker for 5 minutes to incorporate the colorant. Tint strength was then determined by casting 0.1 mm (4 mil) wet thickness films of the untinted and tinted samples on sealed PENOPAC™ panels (Leneta Co., BYK-Gardner USA), allowing the panels to dry under ambient conditions for 30 minutes followed by 30 minutes in a 50° C. (122° F.) oven, and evaluating the dried paint film reflectance values using a DATACOLOR™ 600 spectrophotometer from Datacolor operated with a D65 light source, an 8° angle of reflectance, a large area view using the included diffuse specular reflection sphere and a 10° observation angle. The results were reported as the untinted or tinted L* values. Untinted paint samples were also assessed for contrast ratio (hiding power) according to ASTM method D-2805, using dried drawdowns on Form 3B Leneta charts prepared at a 0.1 mm (4 mil) wet film thickness. Viscosity was evaluated using a BROOKFIELD™ Model No. CAP-1000+L high shear ICI cone and plate viscometer with a No. 2 spindle operated at 900 rpm and 25° C. (77° F.) and reported in poise (P), or evaluated using a BROOKFIELD Model KU-2 Stormer viscometer with its standard paddle blade operated at the fixed machine speed and 25° C. and reported in Krebs units (KU). Storage stability of pigment grind dispersions was evaluated using a TA Instruments Model No. AR2000EX rheometer with a temperature-controlled Peltier plate operated at 25° C. Storage stability of finished coating compositions was evaluated by storing a sealed 0.47 L (1 pint) finished paint sample in a 49° C. (120° F.) oven for one week, removing the sample and inspecting it to ascertain if any separation occurred, and then measuring viscosity using the Stormer viscometer and measuring 60° gloss using a Byk Gardner MICRO-GLOSS handheld gloss meter.

Comparison Example A and Example 1

Pigment grind dispersions were prepared by combining and mixing the ingredients listed below in Table 1, and then combining the dispersions with additional ingredients as indicated to make finished paint formulations. The ingredients in Comparison Example A were combined in a manner like that recommended in a May, 2011 Dow Chemical Co. Technical Data Sheet entitled “EVOQUE Pre-Composite Polymer Technology Technical Data”, but with ingredient and mixing adjustments to make a semigloss paint rather than a flat paint. The ingredients in Example 1 were combined according to the present invention. A number of adjuvants could be freely substituted for ingredients present in minor amounts, so those ingredients were identified generally rather than specifically. No separation was observed following oven aging. Other properties for the Comparison Example A and Example 1 formulations are shown below in Table 2.

TABLE 1 Comparison Example A, Example 1, Ingredient Parts Parts INITIAL MIXTURE (PRE-LETDOWN): Water 35 35 Second polymer (EVOQUE ™ 1140) 202 First polymer (RHOPLEX ™ VSR-1050) 215.5 TiO2 slurry (75 wt. % KRONOS ™ 4310) 269 269 Second polymer (EVOQUE ™ 1140) 202 Stir for 10-20 minutes First polymer (RHOPLEX ™ VSR-1050), 215.5 added w/stirring Defoamer 1 1 Water 10 10 GRIND: Water 100 100 Antisettling agent 2 2 Plasticizer (Dioctyl Maleate) 2 2 Defoamer 0.5 0.5 Dispersant (TAMOL ™ 731A) 4.5 4.5 Extender pigment (IMSIL ™ A-10) 11 11 Preservative 1.6 1.6 Mix in High Speed Disperser 20 minutes LETDOWN (ADDED TO THE GRIND): Surfactant 2 2 Mildewcide 2 2 Mix 2 minutes Add to Pre-Letdown Water (to rinse grind kettle) 40 40 HEUR thickener (AQUAFLOW ™ NHS-310) 40 40 HEUR thickener (AQUAFLOW XLS-525) 8 8 Defoamer 2 2 Opaque polymer (ROPAQUE ™ ULTRA) 65 65 Wetting agent 1 1 TOTAL PARTS 1034 1034

TABLE 2 Comparison Example Example A 1 Tint Strength, tinted L* 83.52 83.51 Tint Strength, untinted L* 98.17 98.21 Contrast Ratio, 0.1 mm (4 mil) 98.88 99.20 Viscosity, P 1.404 1.421 Viscosity, KU 100.2 100.3 Oven-Aged Viscosity, KU 99.7 99.5 Oven-Aged 60° Gloss 42.8 42.4

As shown in Table 2, the method employed to prepare the Example 1 paint formulation provided a nearly identical L* value and an increased contrast ratio compared to the method recommended by the pre-composite polymer supplier. The contrast ratio results were especially promising, and indicate the Example 1 paint should provide very good one coat hiding properties. They also indicate that the paint formulation could be adjusted (e.g., by reducing the titanium dioxide content, second polymer content or both) to provide a paint with tinted L* and contrast ratio values like those in Comparison Example A but at a reduced raw material cost. The formulation may also provide one or more improved product performance or improved product label claims at the formulation levels shown.

Example 2

Using the method of Example 1, a pigment grind dispersion was prepared by combining and mixing the ingredients listed below in Table 3, and then combining the dispersion with additional ingredients as indicated to make a finished paint formulation. No separation was observed following oven aging. Other properties for the Example 2 formulation and for the Comparison Example A formulation are shown below in Table 4:

TABLE 3 Ingredient Example 2, Parts INITIAL MIXTURE (PRE-LETDOWN): Water 35 First polymer (RHOPLEX ™ VSR-1050) 215.5 Add the next 2 ingredients concurrently: TiO2 slurry (75 wt. % KRONOS ™ 4310) 269 Second polymer (EVOQUE ™ 1140) 202 Stir for 10-20 minutes Defoamer 1 Water 10 GRIND: Water 100 Antisettling agent 2 Plasticizer (Dioctyl Maleate) 2 Defoamer 0.5 Dispersant (TAMOL ™ 731A) 4.5 Extender pigment (IMSIL ™ A-10) 11 Preservative 1.6 Mix in High Speed Disperser 20 minutes LETDOWN (ADDED TO THE GRIND): Surfactant 2 Mildewcide 2 Mix 2 minutes Add to Pre-Letdown Water (to rinse grind kettle) 40 HEUR thickener (AQUAFLOW ™ NHS-310) 40 HEUR thickener (AQUAFLOW XLS-525) 8 Defoamer 2 Opaque polymer (ROPAQUE ™ ULTRA) 65 Wetting agent 1 TOTAL PARTS 1034

TABLE 4 Comparison Example Example A 2 Tint Strength, tinted L* 83.52 83.43 Tint Strength, untinted L* 98.17 98.17 Contrast Ratio, 0.1 mm (4 mil) 98.88 98.98 Viscosity, P 1.404 1.412 Viscosity, KU 100.2 100.8 Oven-Aged Viscosity, KU 99.7 100.1 Oven-Aged 60° Gloss 42.8 42.6

As shown in Table 4, the method employed to prepare the Example 2 paint formulation provided a lower L* value but an increased contrast ratio compared to the method recommended by the pre-composite polymer supplier. The hiding power improvement should permit the formulation to be adjusted to reduce the amount of titanium dioxide employed, and may provide one or more improved product performance or improved product label claims at the formulation levels shown.

Having thus described preferred embodiments of the present invention, those of skill in the art will readily appreciate that the teachings found herein may be applied to yet other embodiments within the scope of the claims hereto attached. The complete disclosure of all patents, patent documents, and publications are incorporated herein by reference as if individually incorporated. 

We claim:
 1. A method for making a pigment grind dispersion of titanium dioxide pigment-polymer composite particles, which method comprises: (a1) mixing titanium dioxide particles with an aqueous emulsion containing water-insoluble film-forming first polymer particles that do not by themselves form composite particles with the titanium dioxide particles, and then (b1) adding an aqueous emulsion containing water-insoluble film-forming second polymer particles that adsorb to the titanium dioxide particles, or (a2) circulating an aqueous emulsion containing water-insoluble film-forming first polymer particles that do not by themselves form composite particles with titanium dioxide particles, and (b2) independently and concurrently adding to such first polymer particles (i) a dispersion of titanium dioxide particles and (ii) an aqueous emulsion containing water-insoluble film-forming second polymer particles that adsorb to the titanium dioxide particles, thereby forming titanium dioxide pigment-polymer composite particles that provide improved coating composition opacity compared to a coating composition that does not contain such composite particles.
 2. A method according to claim 1 comprising: (a1) mixing the titanium dioxide particles with the aqueous emulsion containing water-insoluble film-forming first polymer particles that do not by themselves form composite particles with the titanium dioxide particles, and then (b1) adding the aqueous emulsion containing water-insoluble film-forming second polymer particles that adsorb to the titanium dioxide particles.
 3. A method according to claim 2 comprising adding to the first polymer particles an amount of second polymer particles that is less than the saturation level of adsorption when another polymer is not present.
 4. A method according to claim 1 comprising: (a2) circulating the aqueous emulsion containing water-insoluble film-forming first polymer particles that do not by themselves form composite particles with titanium dioxide particles, and (b2) independently and concurrently adding to such first polymer particles (i) the dispersion of titanium dioxide particles and (ii) the aqueous emulsion containing water-insoluble film-forming second polymer particles that adsorb to the titanium dioxide particles.
 5. A method according to claim 4 comprising adding to the first polymer particles an amount of second polymer particles that is less than the saturation level of adsorption when another polymer is not present.
 6. A method according to claim 1 wherein the pigment grind dispersion has a titanium dioxide pigment volume concentration of about 25 to about
 50. 7. A method according to claim 1 wherein the pigment grind dispersion has a titanium dioxide pigment volume concentration of about 27 to about
 45. 8. A method according to claim 1 comprising adding sufficient titanium dioxide particles to the first polymer particles to provide a pigment grind dispersion having a titania composite ratio of about 32 to about
 60. 9. A method according to claim 1 comprising adding sufficient titanium dioxide particles to the first polymer particles to provide a pigment grind dispersion having a titania composite ratio of about 37 to about
 50. 10. A method according to claim 1 wherein the pigment grind dispersion contains about 27 to about 37 weight percent titanium dioxide particles, about 55 to about 14 weight percent first polymer particles and about 18 to about 49 weight percent second polymer particles, based on the dry weights of these particles compared to the total dry weight of ingredients in the pigment grind dispersion.
 11. A method according to claim 1 wherein the first polymer is an acrylic emulsion, ethylene vinyl acetate emulsion, polybutadiene emulsion, polyvinylidene emulsion, styrene acrylic emulsion, vinyl acrylic emulsion, vinyl acetate-vinyl versatate emulsion or mixture thereof.
 12. A method according to claim 1 wherein the second polymer particles include pendant phosphorus groups, acetoacetoxy groups, 1,3-dicarbonyl groups, aldehyde groups, acid groups, amine groups, epoxy groups, isocyanate groups, thiorane groups, isothiocyanate groups, alcohol groups, carbodiimides groups, aziridine groups, haloalkane groups, or halophenyl groups.
 13. A method according to claim 1 wherein the second polymer particles are free of voids.
 14. A method according to claim 1 further comprising adding to the pigment grind dispersion one or more extender particles, dispersing agents and defoamers.
 15. A method according to claim 1 further comprising adding to the pigment grind dispersion one or more coalescents, cosolvents or plasticizers; one or more rheology modifiers; and one or more surfactants or dispersants to provide a coating composition.
 16. A method according to claim 15 further comprising evaluating the tint strength or hiding power of the coating composition and then adding to the coating composition a composite-forming polymer to provide a modified coating composition having increased tint strength or hiding power.
 17. A method according to claim 15 wherein the coating composition contains less than about 5 wt. percent volatile organic compounds. 